Drilling wells for oil and gas production conventionally employs longitudinally extending sections, or so-called “strings,” of drill pipe to which, at one end, is secured a drill bit of a larger diameter. After a selected portion of the bore hole has been drilled, a string of tubular members of smaller diameter than the bore hole, known as casing, is placed in the bore hole. Subsequently, the annulus between the wall of the bore hole and the outside of the casing is filled with cement. Therefore, drilling and casing according to the conventional process typically requires sequentially drilling the bore hole using a drill string with a drill bit attached thereto, removing the drill string and drill bit from the bore hole, and disposing and cementing a casing into the bore hole. Further, often after a section of the bore hole is lined with casing and cemented, additional drilling beyond the end of the casing or through a sidewall of the casing may be desired. In some instances, a string of smaller tubular members, known as a liner string, is run and cemented within previously run casing. As used herein, the term “casing” includes tubular members in the form of liners.
Because sequential drilling and running a casing or liner string may be time consuming and costly, some approaches have been developed to increase efficiency, including the use of reamer shoes disposed on the end of a casing string and drilling with the casing itself. Reamer shoes employ cutting elements on the leading end that can drill through modest obstructions and irregularities within a bore hole that has been previously drilled, facilitating running of a casing string and ensuring adequate well bore diameter for subsequent cementing. Reamer shoes also include an end section manufactured from a material which is readily drillable by drill bits. Accordingly, when cemented into place, reamer shoes usually pose no difficulty to a subsequent drill bit to drill through. For instance, U.S. Pat. No. 6,062,326 to Strong et al. discloses a casing shoe or reamer shoe in which the central portion thereof may be configured to be drilled through. However, the use of reamer shoes requires the retrieval of the drill bit and drill string used to drill the bore hole before the casing string with the reamer shoe is run into the bore hole.
Drilling with casing is effected using a specially designed drill bit, termed a “casing bit,” attached to the end of the casing string. The casing bit functions not only to drill the earth formation, but also to guide the casing into the bore hole. The casing string is, thus, run into the bore hole as it is drilled by the casing bit, eliminating the necessity of retrieving a drill string and drill bit after reaching a target depth where cementing is desired. While this approach greatly increases the efficiency of the drilling procedure, further drilling to a greater depth must pass through or around the casing bit attached to the end of the casing string.
In the case of a casing shoe, reamer shoe or casing bit that is drillable, further drilling may be accomplished with a smaller diameter drill bit and casing string attached thereto that passes through the interior of the first casing string to drill the further section of hole beyond the previously attained depth. Of course, cementing and further drilling may be repeated as necessary, with correspondingly smaller and smaller tubular components, until the desired depth of the wellbore is achieved.
However, drilling through conventional casing and casing associated components (e.g., casing shoes, reamer shoes, casing bits, casing wall, cementing equipment, cement, etc.) often results in damage to the subsequent drill bit and bottom-hole assembly deployed or reduced penetration for at least some period of time. For example, conventional drill bits often include very drilling resistant, robust structures typically manufactured from materials that are difficult to drill through, such as tungsten carbide, polycrystalline diamond, or steel. Furthermore, conventional float shoes, such as casing shoes or reamer shoes, may include casing-associated components that are difficult to drill out, such as rubber or other elastomeric components. Such elastomeric components may, in some situations, cause the drill bit to spin on top of the elastomeric component in the casing component being drilled out instead of being broken up and drilled out, preventing the cutting elements of the drill bit from engaging the borehole surface and inhibiting the drill bit from progressing into the formation. In other situations, conventional drill bits and conventional cutting elements may break the elastomeric components into pieces of sufficient size to plug up the passages for evacuating such cuttings from the drill bit and resulting in what is known as “balling” of the drill bit. For example, the larger pieces of elastomeric components may get caught in the junk slots of a conventional bit, making the conventional bit unable to effectively evacuate cuttings from the bit face, which results in collection of cuttings and debris that inhibit the drill bit from drilling through the remainder of the casing component and progressing efficiently into the formation.
It would be desirable to have a drill bit or tool capable of drilling through casing or casing-associated components, particularly those incorporating elastomers, while at the same time offering the subterranean drilling capabilities of a conventional drill bit or tool employing superabrasive cutting elements.